The present invention relates to the imaging and magnetic resonance arts. It particularly relates to magnetic resonance angiography and will be described with particular reference thereto. However, the invention will also find application in the imaging of other tubular structures and networks in which similar tubular structures and networks are advantageously differentiated.
Angiography relates to the imaging of blood vessels and blood vessel systems. Angiography enables improved surgical planning and treatment, improved diagnosis and convenient non-invasive monitoring of chronic vascular diseases, and can provide an early warning of potentially fatal conditions such as aneurysms and blood clots.
Angiography is performed using a number of different medical imaging modalities, including biplane X-ray/DSA, magnetic resonance (MR), computed tomography (CT), ultrasound, and various combinations of these techniques. Magnetic resonance angiography (MRA) can be performed in a contrast enhanced mode wherein a contrast agent such as Gadolinium-Dithylene-Triamine-Penta-Acetate is administered to the patient to improve vascular MR contrast, or in a non-contrast enhanced mode. Vascular contrast is typically obtained by imaging the flowing blood using MR imaging techniques such as time of flight (TOF), black-blood, phase contrast, T2, or T2* imaging.
The TOF method is prevalent in MRA. A TOF imaging sequence typically includes the steps of exciting a magnetic resonance in a first tissue slice using a 90° RF pulse, followed by applying a 180° phase-refocusing RF pulse to a nearby second slice. There is a time delay correlated to blood flow speed and direction between the 90° and 180° RF pulses. Blood that has flowed from the first slice into the second slice during the time delay experiences both the 90° excitation pulse and the 180° refocusing pulse and so produces a spin echo that is selectively imaged in the TOF technique. TOF as well as most other MRA methods produce a gray scale three-dimensional image in which the blood vessels (or the blood within the blood vessels) appear either brighter (white blood angiographic techniques) or darker (black blood angiographic techniques) than the surrounding tissues.
Analysis and interpretation of the unprocessed gray scale MRA image is complicated by a number of factors. Complexity arises because blood vessel networks in the human body are highly intricate, and a particular image will typically include tortuous or occluded blood vessels, shape variability, “S”-turns, regions of very high blood vessel densities, a wide range of blood vessel diameters, and gaps in blood vessels that complicate tracking. The MRA data acquisition introduces additional complications such as misleading gray scales due to limited dynamic range, partial volume averaging, and the like.
A particularly difficult problem in MRA data interpretation is differentiation of the vascular types, i.e. the arteries and the veins, in the image. Medical personnel often desire two images, namely an arteriogram that displays only the arteries and a venogram that displays only the veins. However, most MRA techniques do not provide an appreciable differentiation between the arterial and the venous vascular sub-systems. In the prior art, vessel type identification is typically achieved using two techniques: selective data acquisition, and post-acquisition image processing to differentiate the arteries and the veins.
Selective data acquisition is usually applied to the time-of-flight (TOF) methods in which a pre-saturation slab is placed either superior or inferior of the imaging volume in order to suppress the signal from either the veins or the arteries. Another selective acquisition method employs ECG gating and the phase contrast method which encodes blood flow velocity during the systolic and diastolic phases of the heart pumping cycle to differentiate the arteriogram and the venogram based on the sign and magnitude of the phase of the generated image data. Both the TOF and phase contrast methods assume that the blood flow direction in the image volume definitively differentiates between venous flow and arterial flow. Temporal differentiation has also been proposed for contrast enhanced MRA. This method uses the difference of circulation time in the arterial and venous vascular systems. By measuring and knowing this time difference for all areas of the imaged volume, the arteriogram and the venogram can be separately acquired.
Selective data acquisition methods have several disadvantages. Scan time is increased because the arteriogram and venogram are separately acquired, effectively doubling the acquisition time. The methods typically perform poorly when the imaging volume includes circulation turning points, as in the case of the head and other body extremities. Near these turning points veins often are improperly imaged in the arteriogram, and/or the venogram has poor contrast, e.g. the inflows are suppressed. Since discrimination of arteries and veins in contrast-enhanced MRA is mainly accomplished by careful selection of the time delay between the first and second passes, the image quality is very sensitive to this timing.
Another disadvantage is that the pre-saturation pulses used in many selective acquisition methods typically greatly increases the specific absorption ratio (SAR). The elevated SAR in pre-saturation techniques is particularly problematic for imaging at high magnetic field strengths. Pre-saturation reduces the effective sampling window which forces a higher sampling bandwidth to be used. Pre-saturation methods are also less effective for black-blood based MRA due to the complexity of the blood flow pattern.
The post-acquisition processing methods include manual identification by a user. Manual identification can use a single slice that preferably intersects the primary or root artery of the arterial system. Alternatively, every slice is analyzed by the user. Manual methods have the disadvantages of slowing down the image analysis process, introducing an element of human error to that process, and requiring operators with sufficient anatomical knowledge of the imaged area of the body to make correct manual vascular identifications.
Automated methods for post-acquisition differentiation of arteries and veins typically include the disadvantage of being restricted in application to only certain MRA imaging methods, for example only phase-contrast MRA or only contrast enhanced MRA. In particular, these post-acquisition processing methods are often incompatible with black-blood based MRA techniques. The black-blood techniques have advantages over white-blood-based MRA in that black-blood techniques typically provide more accurate vascular morphology versus TOF MRA.
The present invention contemplates an improved MRA system and method which overcomes the aforementioned limitations and others.